hiv is not transmitted through casual contact, environmental surfaces or inanimate objects. once exposed to air, hiv becomes inactive and unable to infect.
sharing soap does not constitute a risk.
HIV is transmitted by unprotected vaginal & or anal sex,sharing IV drug works,infected mother to baby,needlestick injuries in a healthcare setting and blood to blood contact from gaping wounds from extreme blood trauma--like car accidents,etc.That's all.
Unless your son has unprotected sex or shares works with her there is no possible way for him to contract HIV.